An apparatus of the aforementioned type is already known from U.S. Pat. No. 3,747,577. This patent discloses an apparatus wherein a temperature-dependent resistor is arranged in a branch of a measuring bridge. The bridge is dimensioned such that the temperature-dependent resistor, as a result of the high current flowing therethrough, is heated to a high temperature which is above the temperature of the flowing medium. The measuring principle of this apparatus is based on the fact that the medium flowing past the resistor draws heat from the resistor the amount of which is dependent on the rate and the density of the flow. The resistance value of the temperature-dependent resistor is thereby changed, so that a measurement of the resistance permits a conclusion to be drawn on the flow rate of the medium. However, it has proved to be advantageous not to determine the resistance directly but to use a control device which maintains the temperature of the temperature-dependent resistor constant with respect to the flowing medium. In order to also take into account the effect of the temperature of the flowing medium, the other branch of the bridge circuit includes another temperature-dependent resistor exposed to the flowing medium which compensates for the influence of temperature. As a measure of the flow rate, the current flowing through the bridge or the voltage drop along a temperature-independent bridge resistor may, for example, be used. Overall, this apparatus is well proven, particularly when used in the measurement of the amount of air supplied to an internal combustion engine, and it is already being used in many automotive vehicles.
It is to be noted, however, that situations may occur in which the apparatus is subjected to extreme conditions and full operational reliability and predetermined measuring accuracy may no longer be assured. An example of such a condition is the special case in which the internal combustion engine of a motor vehicle is switched off with the engine compartment hot while, however, the ignition key remains in a position in which battery voltage continues to be applied to the electrical system of the engine. In spite of the absence of the cooling effect of the flowing medium with the internal combustion engine at a standstill, the apparatus for measuring the flow rate will then continue to be functional. The control member for controlling the flow passing through the temperature-dependent measuring resistor controls the latter to very low values, resulting in a high power loss at the control member itself. Using conventional components, this power loss may cause the control member or parts thereof to reach temperatures of over 200.degree. C. This may particularly reduce the reliability of the individual components of the apparatus.